Endoscopes are commonly used for examining relatively inaccessible objects, for example in body cavities. For evaluating and documenting an object being examined, an externally connected camera may be provided for depicting a part of the body which is under examination. The camera may be a photographic camera, or if a light converter element, for example a CCD, is provided, for converting optical signals into electrical video signals, a video recorder may be used. In order to produce recordings enabling exact diagnosis, optimum illumination of the object examined must be provided for. If a constant quantity of light is supplied by the light source to the object examined, some regions under examination which are highly reflective may be overexposed and others which are less reflective may be underexposed. The light source should, therefore, be automatically adjustable to ensure the optimum quantity of light at variable ranges.
A control circuit for adjusting the intensity of a light source is described, for example, in DE-B 35 09 825, wherein a light projector of adjustable luminous intensity, enables the intensity of the illumination of an object examined to be set according to the distance of the object from the distal end of an endoscope, and limited to a maximum value. This is accomplished by comparing required and actual values of a video signal, taking the maximum permissible amplitude of the video signal as the required value. Such peak value control ensures that the brightest parts of the video picture do not overexpose. The background of the picture is usually too dark, however.
According to DE-B 31 18 341 the pictures are taken by means of a television camera. The quantity of light applied to the light guide is controlled so that the voltage of the video signal is held to an essentially constant level irrespective of changes in the distance between the object under examination and the distal end of the endoscope. Since the mean value of the video signal is used for controlling the brightness of the picture, the object under examination is not always correctly lit.
As described in DE-A 37 43 090, the illuminating light and the brightness of the object depicted are automatically adjusted independently of the size of the picture that can be transmitted by way of the image guide. Alterations in the level of the video signal, resulting from the use of endoscopes having image circles of different diameters and thus different picture ranges, is taken account of by scanning the picture width of the endoscopic image line by line and storing the greatest width of the image as a voltage level by means of a peak value holding circuit and adding it to the actual value of the video signal.
According to DE-A 38 18 125, account is taken of the change in the diameter of the image circle resulting from the use of endoscopes having different diameters and the quantity of light applied to the object examined is adjusted accordingly. This is done by checking the signal levels of pixels of a solid-state video pickup device to determine whether they exceed a specified darkness value, thereby to establish the scanning area corresponding to the area of the cross-section of the image guide. By selection of a correction signal, a light-adjusting signal is generated which adjusts the quantity of light that is fed to the light guide. A square area shown as hatched in the drawing of the reference under discussion, and not the actual image circle area of the endoscope, is used for adjusting the video signal.
US-A-4,628,362, describes a "follow-up control" of an analogue-digital converter for optimum use of the input voltage range of the video signal. An active window is defined in the video picture area. A signal is derived from said active window whereby the full dynamic range of the analogue-digital converter is utilized. The reference under discussion does concern light quantity control and window area detection.